As some here will know, I've been playing about with small brushless motors, intended for use on model aircraft, for some time now. I have been impressed with their ability to deliver a couple of kW or so from a tiny package, but have just acquired something a little larger:

Colossus 7kW.JPG (155.68 KiB) Viewed 10107 times

This motor isn't the biggest in their range, it's just a humble 7kW one, but it still fits neatly in one hand (just.....). Not bad for around £100 all inclusive. I'm probably going to resurrect my RD50 project, rip out the new Mars ME0709 motor and big AXE4834 controller I originally fitted and go for a lightweight build with LiPo batteries. This motor will just fit where the toolbox/airfilter was on the original bike, so that leaves the whole of the engine/gearbox bay open for a battery pack.

In essence, this is a fairly low duty cycle motor, but not, perhaps, as low a duty cycle as an average road vehicle power plant.

The motor in the picture is 75Kv, and is best suited to running at around 60V or more and maybe a couple of kW continuous. I also bought a motor kit (essentially all the motor parts excluding the windings) as I plan on winding a 200Kv motor, using rectangular wire to try and get the best possible copper fill. This should be capable of a higher continuous power, albeit when run at a higher voltage.

The motor is a higher specification version of the cheap Turnigy 80-100, which is available from Hobby City in Hong Kong. The main differences are better quality, including NSK bearings and much better tolerances on the components.

The challenge is building a controller to drive it. I am working on this, using a variation on the small Chinese controllers I've been using on various projects over the past couple of years. Instead of the tiny TO220 package FETs that these controller usually use, I'm using some of these rather larger FETs (shown here with a controller PCB):

I have played with a smaller 480 rpm/V unit and it was very impressive. Got quite hot at 500W but not excessively so. The inside out design is very clever giving a large air gap area towards the periphery of the motor envelope, giving high power density with the lowest speed. I guess the inside stator laminations are the same as a universal motor keeping cost low. Designed for model aircraft the weight is minimised. There is a lot of external field, the outer cylinder not thick enough to link all the flux from the rare earth magnets. So where weight is not critical a further improvement in efficiency might be possible with a thicker cylinder. With a 17 x 4 prop one of these gave 3Kg of thrust at 500W.

Driving them looks complex though. The 3 phase winding needs to be driven with the correct phase sequence and applied voltage for a particular speed. The ESC units sold for them measure the generated emf and pwm modulate the drive to correctly match this emf. Presumably they measure the phase lag between the applied and rotor fields to determine the load and drive enough current to maintain lock. I found open loop as a simple synchronous motor they dropped out of lock very easily.

You're right, driving them with the typical sensorless, back EMF position feedback ESCs that the RC people use doesn't give good results at low speed, high torque as, with the exception of one or two very expensive ESCs, they tend to lose sync and often have poor start-up algorithms. There are some fairly good ESCs about that do work very well though. Those from Castle Creations, particularly their HV160 50 volt, 160 amp high power unit, seem able to give exceptional performance up to motor power levels of around 6 to 8kW though. Several people on the ES forum have used these controllers to drive the cheap Turnigy HXT 80-100 motors (these: http://www.hobbycity.com/hobbyking/stor ... oduct=5142) at power levels of around 5 to 6kW on electric bikes quite successfully. In fact,the stator of that cheap motor is the same as the one on the motors I've just bought - they come from the same Chinese manufacturer.

Driving these motors with a typical cheap Chinese BLDC ebike motor controller, after having fitted Hall position sensors at 120 deg to generate the three phase position feedback signals needed, is dead easy though, and gives excellent performance from a standstill to max rpm at any load. I've been using a 3,250 watt outrunner motor like this (one of these: http://www.hobbycity.com/hobbyking/stor ... oduct=7870) to run my milling machine for a while now and it makes an excellent variable speed, high torque spindle drive motor.

I've also been using smaller, 2,800 watt outrunner motors on my electric boat drives. One of these has successfully driven an 18ft day sailer at hull speed without getting warm. For a controller I used a cheap ($45 inc shipping) electric bicycle BLDC controller and modified the motor by fitting internal Hall sensors between the stator slots.

The BLDC controller circuit board shown in the picture of the FETs is a multifunction, microcontroller based, design that can be programmed via a connection to an ordinary PC, to vary things like max current, phase current, rpm limits, cruise control setting, electric brake settings, regeneration level, low voltage cut-off point etc. The boards, as shown in the picture, are a little bigger than a credit card and cost about $20 each including shipping from China. They are normally fitted with 6 TO220 FETs and used to drive motors up to around 500 watts directly, but can easily be modified to just provide the FET drive signals to external FETs/drivers, which is what I'm doing.

Pretty much all the parts in these outrunner motors are specifically made for them. The stator laminations are typically much larger, with a higher slot count, than brushed motor rotors and tend to use thinner iron laminations to reduce eddy losses from the somewhat higher frequency they operate at (maximum commutation frequency for a typical controller will be around 2000 to 3000Hz). The magnets are usually Neodymium, bonded directly to the steel can. Measurements of external flux leakage I've made shows that the poorer motors do indeed leak a few % of the magnet flux through the thin steel can, but the better motors tend to keep this leakage down below 1%, which is really negligible in terms of performance loss. Typical efficiency at the peak operating point will be around 90% or so. My 3,250 watt motor on the milling machine barely gets warm after an hour or so of hard work, but then it is running in free air. The motor design is such that air is naturally drawn through the intakes in the base mount and around the annular gap between the rotor and the base and expelled through the top of the rotor. Even with no fan the airflow from the milling machine motor is quite substantial, which helps keep the stator cool.

The main issue is getting used to working with motors that need a relatively high ratio reduction drive to deliver good power. I will be spinning my motor at upwards of 8000rpm, probably three times as fast as the much heavier and larger brush motor it will be replacing. The drive design isn't hard, using off-the-shelf HTD toothed belts, but there is a noise issue to be addressed - these big three phase motors howl a bit when running at high speeds.